Hitherto, there is known a sheet finisher which is placed downstream of an image forming apparatus, such as a copier, a printer or an MFP (Multi-Function Peripheral), and performs finishing, such as punching or stitching, on a printed sheet.
Recently, the function of this sheet finisher is diversified, and a sheet folding apparatus (sheet finisher) is developed which has, in addition to punching and stitching functions, a folding function to fold a part of a sheet, and a saddle-stitching and folding function to staple the center of a sheet and then to fold the sheet at the center (for example, JP-A 2002-145516 etc.).
In the sheet finisher having the saddle-stitching and folding function, it becomes possible also to form a booklet (to bind a book) from a plurality of printed sheets.
In the saddle-stitching and folding proposed hitherto, after the center of sheets is stitched with staples or the like, a process is performed in which a fold line is formed on the stitched part by a pair of rollers called fold rollers and folding is performed. A plate-like member called a fold blade is brought into contact with the stitched part of the sheet bundle, and is pressed into a nip of the fold roller pair to form the fold line on the sheet bundle.
However, the period in which the folded part of the sheet bundle is pressed by the nip of the fold rollers is short, and the whole folded part is simultaneously pressed by the nip of the fold rollers, and accordingly, the pressure is dispersed to the whole fold line. Thus, the fold line formed by the fold rollers becomes the fold line to which the pressure is not sufficiently applied. Particularly, when the number of sheets is large, or when a thick sheet is contained in the sheet bundle, the fold line often becomes incomplete.
In order to deal with this problem, a technique is developed in which a roller called a reinforce roller is additionally provided, and the fold line formed by the fold rollers is reinforced by this reinforce roller.
The reinforce roller is constructed of, for example, a pair of rollers movable along the fold line of the sheet bundle. The fold line of the sheet bundle is nipped by the nip of the reinforce roller, and the reinforce roller is moved along the fold line while pressure is applied to the nip, so that the fold line of the sheet bundle is reinforced.
The reinforce roller usually stands by at a home position slightly spaced from the edge of the sheet bundle, and at the time of execution of reinforcing, the reinforce roller is separated from the home position, and reciprocates along the fold line of the sheet bundle. When the reinforcing is ended, the reinforce roller is again returned to the home position. The number of times of reciprocating movement of the reinforce roller for one sheet bundle (hereinafter sometimes referred to as the number of reinforcing times) is not limited to one, but can be two or more.
When the sheet bundle has a certain degree of thickness, the thickness of the fold line part if the number of reinforcing times is two becomes thinner than the fold line part if the number of reinforcing times is one, and the more excellent fold line can be formed. However, when the fold line becomes thin to a certain degree, even if the number of reinforcing times is further increased, the required time is merely increased, and an obtained effect becomes low.
Besides, in general, when a booklet is formed, the outermost sheet of the sheet bundle becomes the cover sheet of the booklet, and a thick paper thicker than an inner sheet (standard paper) is often used for the cover sheet. For the sheet bundle including the thick paper, when the number of reinforcing times is set to be larger than the number of reinforcing times for the sheet bundle including only the standard paper, it becomes easy to obtain an excellent fold line.
When plural booklets are formed, because of the restriction of the containing size or the like of a tray on which the formed booklets are placed, it is standard to provide the upper limit of the number of formed booklets. A booklet formation mode in which the upper limit of the number of formed booklets is provided is called a “standard mode”. On the other hand, when the user desires to continuously form a large number of booklets, a booklet formation mode in which the upper limit of the number of formed booklets is not provided meets the need of the user, and this is called an “unlimited mode”. The “standard mode” and the “unlimited mode” can be selectively switched by, for example, a control panel of an apparatus. When the user selects the “unlimited mode”, a large number of booklets may be formed. In this case, even if the formation time of one booklet is extended slightly, the formation of a booklet having a thinner fold line than a booklet obtained at the time of selecting the “standard mode” meets the need of the user. That is, it is desirable that the number of reinforcing times in the “unlimited mode” is made larger than the number of reinforcing times in the “standard mode”.
As stated above, the suitable number of reinforcing times varies according to the type of a sheet contained in the sheet bundle or the set booklet formation mode. However, if the number of reinforcing times must be changed each time the type of a sheet or the booklet formation condition of the booklet formation mode is changed, the user is urged to perform a complicated operation. Besides, if the number of reinforcing times is set to be unduly large, useless processing time and power consumption are spent. On the other hand, if the number of reinforcing times is set to be unduly small, an excellent fold line can not be obtained.